(1) Field of the Invention
The present invention relates to a packet switch and a switching method, more particularly to a packet switch and a switching method for switching variable length packets by controlling transmission of packets according to the priority of respective packets. More concretely, the present invention relates to a packet switch and a switching method suitable for switching variable length packets in the Internet, as well as intranets, and capable for transmitting the variable length packets by controlling a bandwidth thereof.
(2) Description of the Related Art
The packet communication is a communication method for transmitting and receiving data in a form of a packet comprising a predetermined format of header and a data block obtained by dividing transmission data into blocks each having a proper length, for example, 48 byte to 1.5 Kbytes in length. A packet switch adopts a so-called stored and forward switching method, in which switching of communication data is carried out after the communication data as stored temporarily in the switch, so that it is able to apply a variety of control to the packets passing through the switch.
In the case of the Internet coming into wide use rapidly in recent years, data is communicated as variable length packets with an IP (Internet Protocol) header respectively. Therefore, a packet switching technique is essential to each node for connecting a network to another. An inter-networks connecting apparatus (node apparatus) such as a router is also provided with the packet switching function even if it is not called a “packet switch” actually. Consequently, in the present invention, every network apparatus provided with the packet switching function, including the node apparatus having such a specific name as router, will be referred to as a packet switch.
As a prior art related to such the packet switching, a router is disclosed in Japanese unexamined Patent Publication No. 7-135512. The prior art intends to provide a router with a function for controlling the transfer of received packets according to the priority thereof and a function for controlling the discard of packets performed when a buffer congestion occurs. According to the prior art, packets received by a packet receiving unit are queued in a buffer memory by a priority control unit according to the transmission priority and the discard priority of each packet, then they are transmitted to a packet transmission unit according to the order of the transmission priority. The priority control unit refers to a mapping table based on the transmission priority information and the protocol information included in the header of each received packet to obtain the processing priority and the discard priority corresponding to the received packet, and stores the packet in one of queues prepared corresponding to the discard priority for each processing priority. Stored packets are output sequentially according to the order of descending processing priority. When the free area capacity of the buffer memory goes under a predetermined threshold value, discard control of the stored packets is carried out to avoid congestion. In this discard control, packets with higher discard priority are discarded from queues according to the order of ascending processing priority until the free area capacity of the buffer memory reaches a target threshold value.
In the case of the above related art, only one type of the mapping table is referred for queuing packets. The transmission priority and the discard priority of each packet are defined in the mapping table corresponding to the priority information and the protocol information included in the header of each of received packets. Consequently, in the case of the related art, it is difficult to control a bandwidth for each
packet flow and to provide peculiar packet switching services depending on, for example, the source or destination network of those packets in a node apparatus connecting a plurality of networks such as the Internet.